Currently, in the Internet information age, people are in increasing demand of information. As a carrier of digital information, digital files have become an indispensable part of reading materials in modern times.
An existing method for encrypting a digital file always has various defects. As a result, digital files can be easily copied, which has been a problem troubling both suppliers and copyright holders of the digital files. For instance, the key of a typical digital file is set for a specific user. After having obtained the key, the user can use it to download the digital file at different client terminals for a plurality of times.